The news was a major blow for Mrs May who relied on her former university friend as a close confidant and political advisor. He was the third Cabinet minister to leave government in seven weeks following the recent departures of Michael Fallon and Priti Patel.

The investigation said the allegations by Ms Maltby that he “fleetingly” touched her knee and sent her inappropriate text messages were “plausible” - but it failed to offer a final verdict on the issue.

Ms Maltby revealed to the BBC last night that she told a “long-serving aide of Theresa May” about her allegations in 2016 and added: “I was aware that No 10 knew about it.”

And she told the Daily Telegraph: “I would never, never have told my story about Damian Green if I did not believe I was exposing a pattern of behaviour of which I thought the Prime Minister was personally aware.”

But during a visit to Cyprus today, Mrs May insisted: "The first I learned of these allegations was when Kate Maltby wrote about them in The Times."

A No 10 source said: "The Cabinet Office conducted a thorough investigation into a number of allegations about Damian Green.

"The inquiry findings were published yesterday.

"The PM has made it clear that everyone should be able to work in politics without fear or harassment - that is why she has brought forward a new Code of Conduct for the Conservative Party, and set up a cross-party working group to make recommendations about the Houses of Parliament."

Mrs May is on a trip to RAF Akrotiri in Cyprus - the main airbase for British operations against Islamic State in Iraq and Syria - where she will give a festive address to British troops.

The Prime Minister will also wish service families a happy Christmas at an early years children’s centre before returning to the UK for her own festive celebrations.